Recent scanning tunneling microscopy (STM) observation of U-shaped and
V-shaped spectra (and their mixture) in superconducting Nd
1−xSr
xNiO
2
thin films has been interpreted as presence of two distinct gap symmetries in
this nickelate superconductor [Gu et al., Nat. Comm. 11, 6027 (2020)]. Here,
using a two-band model of nickelates capturing dominant contributions from
Ni-
3dx2−y2 and rare-earth (R)-
5d3z2−r2 orbitals, we show that
the experimental observation can be simply explained within a pairing scenario
characterized by a conventional
dx2−y2-wave gap structure with lowest
harmonic on the Ni-band and a
dx2−y2-wave gap with higher-harmonics on
the R-band. We perform realistic simulations of STM spectra employing
first-principles Wannier functions to properly account for the tunneling
processes and obtain V, U, and mixed spectral line-shapes depending on the
position of the STM tip within the unit cell. The V- and U-shaped spectra are
contributed from Ni and R-bands, respectively, and Wannier functions, in
essence, provide position-dependent weighing factors, determining the spectral
line-shape at a given intra-unit cell position. We propose a phase-sensitive
experiment to distinguish between the proposed
d-wave gap structure and
time-reversal symmetry breaking
d+is gap which yields very similar intra-unit
cell spectra.